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Discussion Starter · #1 ·
I went to a 245/45r18 which is an inch bigger than stock. I'm wondering if this will have a noticeable impact on performance? By my calculations it's about 4.5 % larger than stock so I'd figure it should be minimal but I could be wrong.

My car feels like a dog taking off from a stop but goes really good once it's moving. Punch it at 15mph and it's a totally different car. I had the thought it might be due to having to get a larger tire rolling. I was thinking of going to a 245/40r18 which is back to stock O.D. but maybe I'm wasting my time.
 

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Discussion Starter · #2 ·
Another thought, the 245/40r18 I'm looking at, General Exclaim UHP, is actually a pound heavier than the 245/45r18 I have, does it matter?

If I go to a 225/40r18 it's 3-4 pounds lighter, a hair smaller OD than stock and not so wide so it won't track as bad, maybe that's the tire I should get?

I guess I could get them in try them and see but it's such a pain.
 

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I am running a 235/55/17 and it didn't change anything around to much to make a noticeable difference. I think you will be fine.
 

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The taller tire (larger o.d.) effectively lowers the ratios of all your gears, making them taller. This means the engine is further from the torque sweet spot and requires more speed to get there. That is why the car feels better accelerating from 15mph than from a stop - because the car is in that sweet spot at 15.
 

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go w/ 245-40 they're what you should have went w/ in the first place! 225-40 are and inch shorter than stock and 3/4 inch shorter than 245-40 ,don't worry about weight in tires. look at the weight of wheels! good luck 225-45-18 are another option too. same size as stock.
 

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Discussion Starter · #6 ·
alright, thanks guys


I think I might try both the 225/40 and 245/40 just to see what difference it makes.
 

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Load rating is the "other" number on the tires. The 16in tire on a Fusion is a 91, the 17in is a 93. You should never go lower than the original tires. Since there really isn't much difference in weight between a V6 SEL with 17s and the V6 SE with 16s, you can probably assume the 91 is safe for any Fusion. After scanning some tires on tirerack.com, I do see that there are some XL tires in a 225/40-18 that have a 92 load rating, so those should be ok. But if it's not XL (extra load), then it's only 88, which is much to low to be safe.

Load rating is a function of the amount of air in the tire. So generally a wider tire of the same profile or a taller tire of the same width will have a higher load rating.
 

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Waldo - I am dubious on that since the optional tires for the Mustang does the same thing. The 2007 Mustang GT comes with a 235/55/17 (98W), and Ford offers an optional tire of 235/50/18 (97W). The sprung weight of the vehicle is unchanged, yet the max load of the optional tire has gone down. Ford wouldn't do this if it was dangerous. I figured tire manufacturers built the tires to a spec well beyond their typical usage. :dunno: I bet Ford will release the '08 Sport package Fusion with a 225/45/18. I'd be surprised if they actually put a 245 on the car. Any insight on this?
 

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225/45-18 will be a 91 load rating, so it will be the same as the 16in.

You can never really know what the true minimum load rating is for a vehicle, so you have to assume the lowest rating available on the vehicle is the lowest you should go. A Mustang doesn't weigh a whole lot more than a Fusion, certainly not on the front end, so does it really need a 97 when a Fusion can use a 91? But who would buy a Mustang with 205/60s on it?!
 

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Discussion Starter · #13 ·
just an fyi update,

I'm running the 225/40r18 General UHP now (same load rating as the OEM Michelins), they are .6 " smaller than stock (not a big deal IMO) but are 1.6" smaller than the 245/45r18 I had on before and are about 5 pounds lighter that stock each. What a difference! It's like a new car! The improvement in acceleration is noticeable, braking and handling are much better and it no longer wanders with every groove in the road (which is the reason I didn't go with the wider 245/40r18 to keep stock diameter). So if anyone's considering going with a larger tire, don't! I had no idea how substantial the impact would be, it's not worth it IMHO.
 
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